The market is increasingly interested in halogen free flame retardant additives.
Significant requirements for these products are: high flame retardant effectiveness in reinforced and unreinforced articles, pale intrinsic color, good heat stability, good mechanical and electrical properties and low cost. Besides the halogen containing systems, the halogen-free Flame Retardant (FR) additives used in thermoplastic polymers are:                Inorganic flame retardants belonging, for example, to the metal hydroxide group (Magnesium hydroxide and Aluminium hydroxide); these products have to be used in large amounts to be effective and therefore mechanical properties of the related articles are dramatically damaged.        Melamine derivatives, like melamine cyanurate, melamine (poly)phosphate or melamine pyrophosphate. These products either do not have sufficient thermal stability for overcoming the processing conditions of some thermoplastic polymers or, in case of polyesters, are not fully satisfactory.        Organic derivatives of phosphoric acid such as phosphoric esters (valence state of P=+5) are also well known halogen free flame retardant additives. These products are not very effective and in addition they have several drawbacks: tend to bleed out on exposure to heat, are generally in a liquid state and therefore difficult to handle, have insufficient hydrolytic stability and may affect mechanical and thermal properties of final articles. Although many improvements have been introduced with the new high molecular weight products, such as for example, the condensate phosphate esters having a structure which is derived from Bisphenol A (JP nr 6-228426), however the related polymer articles do not have fully satisfactory performances due to the unbalanced combination of flammability with impact resistance, heat stability and weather resistance.        Red-Phosphorus has been proven to be an effective Flame Retardant additive (WO: 98/30632, WO: 99/27016, JPA: 11-335531, US 2003/0018108) but unfortunately the inherent deep-red colour makes the related polymer product difficult to be used for natural or light coloured applications.        Organic phosphinates, with particular regard to aluminium and zinc phosphinates, have been recently described (U.S. Pat. No. 5,281,637; EP 0699708; EP 0899296; WO 09739053) as a new family of halogen free flame retardant additives for polyesters, especially in combination with nitrogen containing compounds. These products are not fully satisfactory from the cost/performances point of view.        Hypophosphorous acid metal salts, otherwise called hypophosphites or inorganic phosphinates, have been also reported as halogen-free flame retardant additives for flame proofing styrene containing polymers (U.S. Pat. No. 4,618,633). Among these, calcium hypophosphite, is especially effective in glass filled polyester resin compositions made on PBT, when used in combination with nitrogen containing compounds like melamine pyrophosphate, urea phosphate, or polymeric melamine phosphate (U.S. Pat. No. 6,503,969, WO: 098117720; DE: 10137930, EP: 0919591). However, Calcium hypophosphite shows no effect as single additive as well as in combination with melamine cyanurate.        